EP0939418A2 - Feldemissionselektronenquelle und ihr Herstellungsverfahren - Google Patents

Feldemissionselektronenquelle und ihr Herstellungsverfahren Download PDF

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Publication number
EP0939418A2
EP0939418A2 EP99108499A EP99108499A EP0939418A2 EP 0939418 A2 EP0939418 A2 EP 0939418A2 EP 99108499 A EP99108499 A EP 99108499A EP 99108499 A EP99108499 A EP 99108499A EP 0939418 A2 EP0939418 A2 EP 0939418A2
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EP
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Prior art keywords
cathode
silicon oxide
electron source
field
emission electron
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EP99108499A
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English (en)
French (fr)
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EP0939418A3 (de
EP0939418B1 (de
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Keisuke Koga
Yoshikazu Hori
Takehito Yoshida
Yuka Yamada
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Panasonic Corp
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Matsushita Electric Industrial Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J1/00—Details of electrodes, of magnetic control means, of screens, or of the mounting or spacing thereof, common to two or more basic types of discharge tubes or lamps
    • H01J1/02—Main electrodes
    • H01J1/30—Cold cathodes, e.g. field-emissive cathode
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H01J9/022—Manufacture of electrodes or electrode systems of cold cathodes
    • H01J9/025—Manufacture of electrodes or electrode systems of cold cathodes of field emission cathodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J9/00—Apparatus or processes specially adapted for the manufacture, installation, removal, maintenance of electric discharge tubes, discharge lamps, or parts thereof; Recovery of material from discharge tubes or lamps
    • H01J9/02—Manufacture of electrodes or electrode systems
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2201/00—Electrodes common to discharge tubes
    • H01J2201/30—Cold cathodes
    • H01J2201/304—Field emission cathodes
    • H01J2201/30403—Field emission cathodes characterised by the emitter shape
    • H01J2201/30426—Coatings on the emitter surface, e.g. with low work function materials

Definitions

  • the present invention relates to a field-emission electron source such as a cold-emission electron source having prospective applications to an electron-beam-induced laser, a flat solid display device, an ultra-high-speed extremely small vacuum element, and the like. More particularly, it relates to a field-emission electron source using a semiconductor which can be integrated and operated at a low voltage and a method of manufacturing the same.
  • FIGS. 19 to 21 there will be described a first conventional embodiment, which is an extremely small field-emission electron source formed by using a silicon substrate and a method of manufacturing the same disclosed in European Laid-Open Patent Publication No. 637050A2.
  • a silicon oxide film 102 is formed by thermal oxidation on the (100) crystal plane of a silicon substrate 101 made of a silicon crystal, followed by the formation of a photoresist film 103 on the silicon oxide film 102.
  • the photoresist film 103 is processed by photolithography to form disk-shaped etching masks 103A each having a diameter of about 1 ⁇ m. Subsequently, the pattern of the etching masks 103A is transferred to the silicon oxide film 102 by dry etching for forming disk-shaped elements 102A, followed by the removal of the etching mask 103A.
  • anisotropic dry etching is performed with respect to the silicon substrate 101 by using the disk-shaped elements 102A as a mask, thereby forming cylindrical elements 104A made of the silicon substrate 101 under the disk-shaped elements 102A.
  • crystal anisotropic etching is performed with respect to the cylindrical elements 104A, thereby forming hourglass elements 104B each composed of a pair of truncated cones with their top surfaces joined to each other and having a side surface including the (331) crystal plane, as shown in FIG. 19(d).
  • a thin first thermal oxide film 105 is formed over the surfaces of the hourglass elements 104B and of the silicon substrate 101. Then, anisotropic dry etching is performed with respect to the silicon substrate 101 by using the disk-shaped elements 102A as a mask, thereby transforming the hourglass elements 104B into cylindrical elements 104C with respective hourglass heads.
  • a second thermal oxide film 106 is formed over the surfaces of the cylindrical elements 104C with respective hourglass heads and of the silicon substrate 101 so that tower-shaped cathodes 107 each having a sharply tapered tip portion and an extremely small diameter are formed inside the cylindrical elements 104C with respective hourglass heads.
  • insulating films 108 and metal films 109 are successively formed by vapor deposition on the disk-shaped elements 102A as well as on the silicon substrate 101 around the disk-shaped elements 102A.
  • wet etching is performed with respect to the second thermal oxide film 106, thereby removing the disk-shaped elements 102A in conjunction with the insulating films 108 and metal films 109 deposited thereon. This exposes the tower-shaped cathodes 107, while forming the metal film 109 into a withdrawn electrode 109A having an inner diameter equal to the diameter of the disk-shaped element 102A.
  • Japanese Laid-Open Patent Publication HEI 6-231675 proposes not only the approach of reducing the size of the cathode and improving the structure thereof described in the first conventional embodiment but also an attempt to improve the performance of the cathodes by selectively depositing the low-work-function material on the tip portions of the cathodes.
  • the formation of the cathodes is followed by oblique vapor deposition for selectively forming the low-work-function material on the surfaces of the tip portions of the cathodes. Thereafter, a thermal treatment is performed for silicidization.
  • the manufacturing method intends a great increase in the efficiency of electron emission by lowering the work function at the tip portion of the cathode.
  • a thermal oxide film 106 formed with an opening corresponding to a region in which the cathode is to be formed is deposited on an n-type silicon substrate 101.
  • an extremely small cathode 107 made of silicon is formed on the thermal oxide film 106.
  • the surface of the cathode 107 has been anodized by means of an anodizing apparatus as shown in FIG. 23, whereby a porous layer 107a has been formed therein.
  • the treating agent In the treating agent, a specified current is allowed to flow between the cathode and anode electrodes 113 and 114 provided on both sides of the sample holder 111, while radiation from an excimer lamp is applied to the sample 112, thereby anodizing the surface of the cathode 107.
  • the composition of the treating agent, the amount of current flowing through the treating agent, and irradiation conditions for the excimer lamp are optimized to form the porous layer 107A made of silicon and having a desired configuration and thickness in the surface region of the cathode 107.
  • the porous layer 107a formed in the surface region of the cathode 107 has numerous rods each having a diameter on the order of nanometers, which have been formed through the formation of numerous holes each having a diameter on the order of nanometers in the porous layer 107a.
  • the numerous rods effectively serve as current emitting sites. This changes the cathode from point-emission type with one emitting site to surface-emission type with numerous emitting sites, resulting in an increased number of electron emitting sites and improved current-emitting property of the cathode.
  • the field-emission electron source according to the first conventional embodiment is operable at a low voltage due to the tower-shaped cathode having a sharply tapered tip portion with an extremely small diameter, it presents the following problem.
  • the current emitted from the cathode is greatly affected by vacuum atmosphere and the surface state of the tip portion of the cathode during operation, so that the physical property, such as work function, of the surface of the current emitting element is changed during current emission, causing a significant change in operating current.
  • the cause of the unsatisfied requirement may be ions resulting from collisions between emitted electrons and a residual gas around the cathode during operation. The resulting ions collide with the tip portion of the cathode and thereby change the surface state of the tip portion of the cathode.
  • the tower-shaped cathode shown in the second embodiment which has a surface coating film formed selectively of the low-work-function material on the tip portion thereof, has the following problem that, since the current emitted from the cathode flows intensively to the bottom portion of the tower-shaped cathode, high Joule heat is generated in the bottom portion of the tower when operation is performed with a large current.
  • a current exceeding a maximum permissible value determined by the substrate resistance and the cross-sectional area of the tower is allowed to flow, the temperature of the cathode is raised by the generated Joule heat. If a temperature exceeding the melting point of the material composing the cathode is reached, the melted cathode may destroy the whole device.
  • the maximum value of the current that can be allowed to flow to the cathode is lowered with increasing miniaturization of the cathode for reducing the operating current, which presents a large obstacle to operation with a large current.
  • the second conventional embodiment has the possibility of solving the problem because of the low-work-function material formed selectively on the tip portion of the cathode by oblique vapor deposition and subjected to the thermal treatment for forming a silicide film on the tip portion of the cathode, it also presents the following problem since the formation of the silicide film involves the process of forming the metal film by vapor deposition and the subsequent reaction process by thermal treatment.
  • a film formed by vapor deposition is apt to have an unequal thickness over a wafer since a source of vapor is a point source.
  • the subsequent process of forming a silicide film by thermal treatment utilizes a crystal reaction at the interface between the deposited metal and the underlying silicon substrate, the rate of the silicidization process and the quality of the resulting silicide film are likely to vary due to the unequal film thickness and non-uniform temperature, which causes a problem in the formation of the tip portion of the cathode that should be microstructured.
  • the radius of curvature of the tip portion is a parameter exerting a particularly great influence on the characteristics of the operating voltage during electron emission. If coefficients of electrostatic focusing are calculated for individual cathodes on the assumption that the structures of the cathodes are the same except for the radii of curvature of the tip portions, the coefficient of electrostatic focusing calculated for the cathode having the tip portion with the radius of curvature of 2 nm is double the coefficient of electrostatic focusing calculated for the cathode having the tip portion with the radius of curvature of 10 nm.
  • the radius of curvature of the tip portion of the cathode easily varies by about 10 nm under the influence of variations in the silicide process, resulting in varied device characteristics, which presents a serious problem to the practical applications.
  • the field-emission electron source according to the third conventional embodiment has the porous layer formed on the surface of the cathode, the number of electron emitting sites is increased with the changing of the cathode from point-emission type to surface-emission type. As a result, the electron emitting property of the cathode is improved to a certain degree, but not to a degree satisfactory for the practical applications.
  • the field-emission electron source according to the third conventional embodiment has the porous layer formed by anodization on the surface of the cathode, improvements have been intended in device characteristics such as operation at a low voltage and an increased current.
  • a thick porous layer having a thickness of several hundreds of nanometers should be formed on the surface of the cathode.
  • a porous layer having a thickness of 470 nm is formed on the surface, there has been observed the effect of increasing the current which is five to ten times as large as the current flowing in the case where no porous layer is formed.
  • the formation of a thick porous layer having a thickness of several hundreds of nanometers on the surface of the cathode degrades the configuration of the tip portion of the cathode.
  • the critical requirements placed on the performance of the field-effect electron source for the practical applications thereof includes uniform electron emission and stable device characteristics in addition to a reduced operating voltage and an increased current, the radius of curvature of the tip portion of the cathode varies in the field-emission electron source according to the third conventional embodiment, which in turn causes the problems of non-uniform electron emission and unstable device characteristics.
  • the object of the present invention is to ensure positive emission of electrons and reduce variations in the characteristics of the operating voltage during electron emission even when slight variations are observed in the configurations of the tip portions of the cathodes.
  • the high-concentration impurity layer is formed in the surface region of the cathode, so that electrons are positively emitted from the surface region of the cathode, resulting in lower power consumption of the resulting device.
  • the cathode preferably has a tower-like configuration and the high-concentration impurity layer is formed indiscretely in the surface region of the cathode and in a surface region of a portion of the substrate exposed in the opening of the withdrawn electrode.
  • the high-concentration impurity layer preferably has a sheet resistivity of 10k ⁇ or less. The arrangement remarkably improves the electron emitting property and thereby greatly reduces the power consumption of the resulting device.
  • the formation of the cathode and of the withdrawn electrode having the opening surrounding the cathode is followed by the formation of the high-concentration impurity layer, so that the high-concentration impurity layer is formed selectively in the surface region of the cathode. Accordingly, the second field-mission electron source can be manufactured simply with high reproducibility.
  • the high-concentration-impurity-layer forming step preferably includes the steps of: forming a deposit film containing an impurity element over a surface of the cathode; and forming the high-concentration impurity layer in a surface region of the cathode by causing solid phase diffusion of the impurity element contained in the deposit film into the surface region of the cathode.
  • the high-concentration impurity layer can be formed positively and selectively in the surface region of the cathode.
  • the high-concentration-impurity-layer forming step preferably includes the step of forming the high-concentration impurity layer in a surface region of the cathode by introducing an impurity element into the surface region of the cathode by ion implantation.
  • the high-concentration impurity layer can be formed positively and selectively in the surface region of the cathode.
  • FIG. 1(a) shows a cross-sectional structure of the electron source taken along the line I-I of FIG. 1(b) and FIG. 1(b) shows a plan structure thereof.
  • a withdrawn electrode 19A is formed on a silicon substrate 11 made of a silicon crystal with intervention of an insulating film consisting of an upper silicon oxide film 18A and a lower silicon oxide film 16A each having circular openings corresponding to respective regions in which cathodes are to be formed, which have been arranged to form an array.
  • the diameter of the opening of the withdrawn electrode 19A is smaller than the diameters of the respective openings of the upper and lower silicon oxide films 18A and 16A so that the circumferential surfaces of the openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • each of the cathodes 17 has a sharply tapered tip portion with a radius of 2 nm or less, which has been formed by crystal anistropic etching and thermal oxidation process for silicon.
  • a thin surface coating film 20 made of a low-work-function material composed of a high-melting-point metal material or of a compound material thereof.
  • a high-melting-point metal material such as Cr, Mo, Nb, Ta, Ti, W, or Zr or a compound material such as a carbide, nitride, or silicide of the high-melting-point material can be used appropriately. This improves the physical and chemical properties of the surface of the cathode 17.
  • a TiN film is formed as the surface coating film 20 by sputtering on the surface of the cathode 17 to a thickness of about 10 nm, the sharply tapered configuration of the tip portion of the underlying cathode 17 is substantially reproduced so that the cathode 17 coated with the TiN film also sharply tapered is implemented.
  • the work function of TiN is estimated to be about 2.9 eV, while the work function of silicon is about 4.8 eV, so that a considerable reduction has been achieved in the work function at the surface of the tip portion of the cathode 17. Accordingly, a starting voltage required for electron emission can be reduced significantly.
  • the foregoing coating materials composing the surface coating film 20 are considered to have more stable chemical properties than the chemical properties of silicon, the use of the coating materials may also be effective in improving the stability of the current flowing during electron emission.
  • the insulating film consisting of the upper and lower silicon oxide films 18A and 16A recessed relative to the withdrawn electrode 19A insulation provided between the cathode 17 and the withdrawn electrode 19A is excellently maintained even when the surface coating film 20 is formed over the entire surface of the cathode 17 and therefore no short-circuit failure occurs.
  • the recessed configuration is extremely effective in improving the production yield of the device and the reliability of the operation thereof.
  • the first silicon oxide film 12 is formed by thermal oxidation on the (100) crystal plane of the silicon substrate 11 made of a silicon crystal, followed by the deposition of a photoresist film 13 on the first silicon oxide film 12.
  • the photoresist film 13 is subjected to photolithography for forming disk-shaped resist masks 13A each having a diameter of about 0.5 ⁇ m.
  • anisotropic dry etching is performed with respect to the first silicon dioxide film 12 by using the resist masks 13A, thereby transferring the pattern of the resist masks 13A to the first silicon dioxide film 12 and forming silicon oxide masks 12A therefrom.
  • the removal of the resist mask 13A is followed by anisotropic etching performed with respect to the silicon substrate 11 by using the silicon oxide masks 12A, whereby cylindrical elements 14A are formed on the surface of the silicon substrate 11.
  • wet etching is performed with respect to the cylindrical elements 14A by using an etching agent having crystal anisotropy, such as an aqueous solution of ethylenediamine and pyrocatechol, thereby forming hourglass elements 14B each having a side surface including the (331) crystal plane and constricted in the middle.
  • an etching agent having crystal anisotropy such as an aqueous solution of ethylenediamine and pyrocatechol
  • second silicon oxide films 15 each having a reduced thickness of about 10 nm are formed on the sidewalls of the hourglass elements 14B by thermal oxidation to protect the constricted portions of the hourglass elements 14B.
  • anisotropic dry etching is performed with respect to the silicon substrate 11 by using again the silicon oxide masks 12A to vertically etch the silicon substrate 11, thereby forming cylindrical elements 14C with respective hourglass heads on the surface of the silicon substrate 11, as shown in FIG. 8(b).
  • a third silicon oxide film 16 having a thickness of about 100 nm is formed by thermal oxidation over the surfaces of the cylindrical elements 14C with respective hourglass heads and of the silicon substrate 11, thereby forming cathodes 17 inside the cylindrical elements 14C with respective hourglass heads.
  • the third silicon oxide film 16 thus formed over the surfaces of the cylindrical elements 14C with respective hourglass heads is for sharply tapering the tip portions of the cathodes 17 and enhancing the insulating property of an insulating film underlying the withdrawn electrode, which will be described later.
  • the silicon oxide film formed by thermal oxidation is superior in film quality to a silicon oxide film formed by another method such as vapor deposition, so that it has high insulation resistance. As a result, there can be formed a highly reliable device exhibiting an excellent insulating property during the application of a voltage to the withdrawn electrode, which will be described later.
  • a fourth silicon oxide film 18 used as an insulating film and a conductive film 19 used as the withdrawn electrode are successively deposited by vacuum vapor deposition with the silicon oxide masks 12A interposed therebetween.
  • ozone gas is introduced so as to form a high-quality silicon oxide film excellent in insulating property.
  • the use of a Nb metal film as the conductive film 19 enables the formation of a uniform withdrawn electrode during a lift-off process, which will be described later.
  • wet etching is performed by using a buffered hydrofluoric acid in an ultrasonic atmosphere to selectively remove the sidewall portions of the cathodes 17 and the silicon oxide masks 12A, thereby lifting off the conductive film 19 deposited on the silicon oxide masks 12A, while exposing the withdrawn electrode 19A having small openings and the cathodes 17.
  • the duration of wet etching is controlled such that the third and fourth silicon oxide films 16 and 18 are overetched. In this manner, the circumferential surfaces of the openings of the upper and lower silicon oxide films 18 and 16 are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • surface coating films 20 made of a coating material composed of a metal material having a low work function or a compound material of the metal material is formed all over by sputtering, resulting in the field-emission electron source according to the first embodiment.
  • surface coating films 20 excellent in coating property can be formed on the cathodes 17 even when a coating material composed of a high-melting-point metal material or a compound material thereof is used.
  • the thicknesses of the surface coating films 20 By adjusting the thicknesses of the surface coating films 20 to be 10 nm or less, there can be obtained a surface configuration faithfully reflecting the structures of the underlying cathodes 17. As a result, cathodes 17 each having a microstructured tip portion on the order of nanometers can be obtained even after the formation of the surface coating films 20.
  • collimate sputtering used to form the surface coating films 20 imparts excellent directivity to deposition so that the surface coating films 20 are formed uniformly not only on the surfaces of the cathodes 17 but also on the bottom portions of the silicon substrate 11 exposed in the openings of the withdrawn electrode 19A. Consequently, it becomes possible to apply the surface coating process to a microstructured device having the prospect of operating at a lower voltage, which is advantageous in enhancing the performance of the device.
  • the foregoing manufacturing method also offers the advantage of excellently uniform and reproducible process and enables the formation of an extremely small field-emission electron source array at a high density with high precision.
  • the coating material composed of the high-melting-point metal material or a compound material thereof each having a low work function can be formed with high accuracy on the surfaces of the cathodes 17 made of silicon, the operating voltage for electron emission can be lowered to a value much lower than reached conventionally.
  • FIG. 2(a) shows a cross-sectional structure taken along the line II-II of FIG. 2(b) and FIG. 2(b) shows a plan structure.
  • a withdrawn electrode 19A is formed on a silicon substrate 11 made of a silicon crystal with intervention of an insulating film consisting of an upper silicon oxide film 18A and a lower silicon oxide film 16A each having circular openings corresponding to respective regions in which cathodes are to be formed, which have been arranged to form an array.
  • the diameter of the opening of the withdrawn electrode 19A is smaller than the diameters of the respective openings of the upper and lower silicon oxide films 18A and 16A so that the circumferential surfaces of the respective openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • each of the cathodes 17 has a sharply tapered tip portion with a radius of 2 nm or less, which has been formed by crystal anisotropic etching and thermal oxidation process for silicon.
  • a high-concentration impurity layer 22 having the same conductivity type as that of the silicon substrate 11 and containing an impurity at a concentration higher than the impurity concentration of the silicon substrate 11.
  • the efficiency of electron emission at the tip portion of the cathode 17 can be increased significantly. Consequently, a starting voltage required to emit a specified quantity of electrons can be reduced significantly or the quantity of electrons that can be emitted at a specified starting voltage can be increased significantly.
  • a first silicon oxide film 12 is formed by thermal oxidation on the (100) crystal plane of the silicon substrate 11 made of a silicon crystal, followed by the deposition of a photoresist film 13 on the first silicon oxide film 12.
  • the photoresist film 13 is subjected to photolithography for forming disk-shaped resist masks 13A each having a diameter of about 0.5 ⁇ m. Subsequently, anisotropic dry etching is performed with respect to the first silicon oxide film 12 by using the resist masks 13A, thereby transferring the pattern of the resist masks 13A to the first silicon oxide film 12 and forming silicon oxide masks 12A therefrom.
  • the removal of the resist masks 13A is followed by anisotropic dry etching performed with respect to the silicon substrate 11 by using the silicon oxide masks 12A, thereby forming cylindrical elements 14A on the surface of the silicon substrate 11.
  • wet etching is performed with respect to the cylindrical elements 14A by using an etching agent having crystal anisotropy, such as an aqueous solution of ethylene diamine and pyrocatechol, thereby forming hourglass elements 14B each having a side surface including the (331) crystal plane and constricted in the middle.
  • an etching agent having crystal anisotropy such as an aqueous solution of ethylene diamine and pyrocatechol
  • second silicon oxide films 15 each having a reduced thickness of about 10 nm are formed on the sidewalls of the hourglass elements 14B by thermal oxidation to protect the constricted portions of the hourglass elements 14B.
  • anisotropic dry etching is performed with respect to the silicon substrate 11 by using again the silicon oxide masks 12A to vertically etch the silicon substrate 11, thereby forming cylindrical elements 14C with respective hourglass heads on the surface of the silicon substrate 11, as shown in FIG. 11(b).
  • a third silicon oxide film 16 having a thickness of about 100 nm is formed by thermal oxidation over the surfaces of the cylindrical elements 14C with respective hourglass heads and of the silicon substrate 11, thereby forming cathodes 17 inside the cylindrical elements 14C with respective hourglass heads.
  • the third silicon oxide film 16 thus formed on the surfaces of the hourglass elements 14C is for sharply tapering the tip portions of the cathodes 17 and enhancing the insulating property of an insulating film underlying the withdrawn electrode, which will be described later.
  • the silicon oxide film formed by thermal oxidation is superior in film quality to a silicon oxide film formed by another method such as vapor deposition, so that it has high insulation resistance. As a result, there can be formed a highly reliable device exhibiting an excellent insulating property during the application of a voltage to the withdrawn electrode, which will be described later.
  • a fourth silicon oxide film 18 used as an insulating film and a conductive film 19 used as the withdrawn electrode are successively deposited by vacuum vapor deposition with the silicon oxide masks 12A interposed therebetween.
  • ozone gas is introduced so as to form a high-quality silicon oxide film excellent in insulating property.
  • the use of a Nb metal film as the conductive film 19 enables the formation of a uniform withdrawn electrode during a lift-off process, which will be described later.
  • wet etching is performed by using a buffered hydrofluoric acid in an ultrasonic atmosphere to selectively remove the sidewall portions of the cathodes 17 and the silicon oxide masks 12A, thereby lifting off the conductive film 19 deposited on the silicon oxide masks 12A, while exposing the withdrawn electrode 19A having small openings and the cathodes 17.
  • the duration of wet etching is controlled such that the third and fourth silicon oxide films 16 and 18 are overetched. In this manner, the circumferential surfaces of the openings of the upper and lower silicon oxide film 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • a glass layer containing an impurity element at a high concentration such as a phosphorus glass layer 21, is deposited over the entire surface of the silicon substrate 11 including the cathodes 17, followed by rapid thermal application (RTA) for performing a proper thermal treatment with respect to the phosphorus glass layer 21.
  • RTA rapid thermal application
  • the thermal treatment causes solid phase diffusion of the impurity element contained in the phosphorus glass layer 21 into the surface region of the cathodes 17, so that the high-concentration impurity layer 22 is formed in the surface regions of the cathodes 17, as shown in FIG. 12(c).
  • the high-concentration impurity layer 22 having a sheet resistivity of 10 k ⁇ or less is formed uniformly at a depth on the order of several tens of nanometers from the surface of the cathode 17. Thereafter, the phosphorus glass layer 21 is removed, resulting in the field-emission electron source according to the second embodiment.
  • the manufacturing method of the second embodiment has formed the high-concentration impurity layer 22 by solid phase diffusion using the phosphorus glass layer 21, the high-concentration impurity layer 22 may also be formed otherwise by introducing the impurity element into the surface of the cathode 17 by ion implantation with low energy and activating the impurity element by a thermal treatment.
  • the high-concentration impurity layer 22 having a depth on the order of several tens of nanometers can be formed uniformly in the surface region of the cathode 17 by introducing phosphorus as the impurity element by ion implantation with acceleration energy of, e.g., 5 keV.
  • the high-concentration impurity layer 22 can be formed uniformly in the surface region of the cathode 17 with high productivity. Since the impurity concentration may be increased at the tip portion of the cathode 17, the efficiency of electron emission is remarkably improved, which achieves a significant reduction in starting voltage required to emit a specified quantity of electrons or a significant increase in the quantity of emitted electrons at a specified starting voltage.
  • the methods of manufacturing the field-emission electron sources according to the first and second embodiments have used crystal anisotropic etching and thermal oxidation process to form the cathodes 17 and the withdrawn electrode 19A on the (100) crystal plane of the silicon substrate 11 made of a silicon crystal and thereby implemented the sharply tapered tip portions of the cathodes 17, it is also possible to alternatively adopt a method in which a polysilicon film is formed at low temperature on a glass substrate and a thermal treatment, such as laser annealing, is performed with respect to prescribed regions of the polysilicon film in which the field-emission electron sources are to be formed, thereby crystallizing the polysilicon film in the prescribed regions.
  • the method enables the formation of an array of field-emission electron sources occupying a large area on the low-cost glass substrate.
  • a substrate made of another semiconductor material such as a compound semiconductor of GaAs or the like may be used.
  • first and second embodiments have used the tower-shaped cathode 17 and the withdrawn electrode 19 having circular openings
  • the configurations of the cathode 17 and of the withdrawn electrode 19 are not limited thereto.
  • a description will be given to an embodiment using a cathode 17 having a configuration different from the configuration used in the first and second embodiments.
  • FIG. 3(a) shows a cross-sectional structure taken along the line III-III of FIG. 3(b) and FIG. 3(b) shows a plan structure.
  • a withdrawn electrode 19A is formed on a silicon substrate 11 made of a silicon crystal with intervention of an insulating film consisting of an upper silicon oxide film 18A and a lower silicon oxide film 16A each having openings corresponding to respective rectangular regions in which cathodes are to be formed, which have been arranged to form an array.
  • the length of each side of the openings of the withdrawn electrode 19A is smaller than the length of each corresponding side of the openings of the upper and lower silicon oxide films 18A and 16A so that the circumferential surfaces of the respective openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • cathodes 18 of wedged structure In the openings of the upper and lower silicon oxide films 18A and 16A and of the withdrawn electrode 19A, there are formed cathodes 18 of wedged structure.
  • a high-concentration impurity layer 22 having the same conductivity type as that of the silicon substrate 11 and containing an impurity at a concentration higher than the impurity concentration of the silicon substrate 11.
  • FIG. 4(a) shows a cross-sectional structure taken along the line IV-IV of FIG. 4(b) and FIG. 4(b) shows a plan structure.
  • a withdrawn electrode 19A is formed on a silicon substrate 11 made of a silicon crystal with intervention of an insulating film consisting of upper and lower silicon oxide films 18A and 16A each having openings corresponding to respective circular regions in which cathodes are to be formed, which have been arranged to form an array.
  • the diameter of the opening of the withdrawn electrode 19A is smaller than the diameters of the respective openings of the upper and lower silicon oxide films 18A and 16A so that the circumferential surfaces of the openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • a shallow high-concentration impurity layer 22 having the same conductivity type as that of the silicon substrate 11 and containing an impurity at a concentration higher than the impurity concentration of the silicon substrate 11.
  • FIG. 5(a) shows a cross-sectional structure taken along the line V-V of FIG. 5(b) and FIG. 5(b) shows a plan structure.
  • a withdrawn electrode 19A is formed on a silicon substrate 11 made of a silicon crystal via an insulating film consisting of upper and lower silicon oxide films 18A and 16A each having circular openings corresponding to regions in which cathodes are to be formed, which have been arranged to form an array.
  • the diameter of the opening of the withdrawn electrode 19A is smaller than the diameters of the respective openings of the upper and lower silicon oxide films 18A and 16A so that the circumferential surfaces of the openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • each of the cathodes 17 has a sharply tapered tip portion with a radius of 2 nm or less, which has been formed by crystal anisotropic etching and thermal oxidation process for silicon.
  • the portion of the silicon substrate 11 exposed in the respective openings of the upper and lower silicon oxide films 18A and 16A and the cathode 17 have their surfaces coated with a surface coating layer 23 composed of an ultra-fine particulate structure formed by laser ablation.
  • a material composing the surface coating layer 23 preferably has a low work function so that electrons are emitted positively.
  • ultra-fine particles composing the surface coating layer 23 silicon particles each having a diameter on the order of nanometers, i.e., a diameter of 10 nm or less are preferred in terms of the efficiency of electron emission.
  • the ultra-fine particles composing the surface coating layer 23 are deposited in a single or several layers.
  • silicon particle has a diameter of about 10 nm
  • a single layer of silicon particles is sufficient.
  • silicon particles are preferably deposited in two or three layers, as shown in FIG. 18(a).
  • FIG. 18(b) shows the cross-sectional structure of a porous layer 107a made of silicon and formed by anodization (etching) over the surface of the cathode 107 in the field-emission electron source according to the third conventional embodiment.
  • the tip portion of the cathode 17 has an obtuse configuration, resulting in an increased and varied radius of curvature. Consequently, device characteristics have varied in the third conventional embodiment, which renders device design difficult and reduces the reliability of the resulting device, presenting a serious problem to the practical applications.
  • the surface coating layer 23 composed of the ultra-fine particulate structure has been formed on the surface of the cathode 17 so that the tip portion of the cathode 17 is prevented from having an obtuse configuration. Accordingly, the radius of curvature of the tip portion is not increased nor varied, resulting in easier device design and higher reliability of the device.
  • the radius of curvature of the tip portion is a parameter exerting a particularly great influence on the characteristics of the operating voltage during electron emission. If the relationship between the radius of curvature and a coefficient of electrostatic focusing is simulated on the assumption that the conditions other than the radius of curvature are the same, the coefficient of electrostatic focusing at the tip portion with the radius of curvature of 2 nm is approximately double the coefficient of electrostatic focusing at the tip portion with the radius of curvature of 10 nm. In other words, the coefficient of electrostatic focusing is approximately halved when the radius of curvature of the tip portion of the cathode is increased from 2 to 10 nm. Thus, the fundamental characteristics of the device such as operating current and operating voltage are greatly changed by a slight change on the order of nanometers in the radius of curvature of the tip portion of the cathode.
  • the surface coating layer 23 composed of the ultra-fine particulate structure has been formed over the surface of the cathode 17, so that variations in the quantity of emitted electrons are eliminated by the averaging effect of numerous ultra-fine particles. This provides an extremely stable electron emitting property, while a drastic increase in the quantity of emitted electrons is suppressed, thereby eliminating the problem of the destroyed cathode resulting from an extraordinary increase in the quantity of emitted electrons.
  • the number of electron emitting sites in the surface coating layers 23 composed of the ultra-fine particulate structure in the fifth embodiment is much larger than the number of electron emitting sites in the porous layer 107a in the third conventional embodiment. Therefore, an extremely large quantity of electrons are emitted from the surface coating layer 23 over the cathode 17, while a more stable current flows from the cathode 17 during electron emission since the apparent work function is less likely to change.
  • the operating current and operating voltage can be reduced, while device characteristics such as operating current and operating voltage do not vary.
  • the ultra-fine particulate structure composing the surface coating layer 23 may be made of a low-work-function material other than silicon, such as diamond, DLC (Diamond Like Carbon), or ZrC.
  • a first silicon oxide film 12 is formed by thermal oxidation on the (100) crystal plane of the silicon substrate 11 made of a silicon crystal, followed by the deposition of a photoresist film 13 on the first silicon oxide film 12.
  • the photoresist film 13 is subjected to photolithography for forming disk-shaped resist masks 13A each having a diameter of about 0.5 ⁇ m.
  • anisotropic dry etching is performed with respect to the first silicon oxide film 12 by using the resist masks 13A, thereby transferring the pattern of the resist masks 13A to the first silicon oxide film 12 and forming silicon oxide masks 12A therefrom.
  • the removal of the resist masks 13A is followed by anisotropic dry etching performed with respect to the silicon substrate 11 by using the silicon oxide masks 12A, thereby forming cylindrical elements 14A on the surface of the silicon substrate 11.
  • wet etching is performed with respect to the cylindrical elements 14A by using an etching agent having crystal anisotropy, such as an aqueous solution of ethylene diamine and pyrocatechol, thereby forming hourglass elements 14B each having a side surface including the (331) crystal plane and constricted in the middle.
  • an etching agent having crystal anisotropy such as an aqueous solution of ethylene diamine and pyrocatechol
  • second silicon oxide films each having a reduced thickness of about 10 nm are formed on the sidewalls of the hourglass elements 14B by thermal oxidation to protect the constricted portions of the hourglass elements 14B.
  • anisotropic dry etching is performed with respect to the silicon substrate 11 by using again the silicon oxide masks 12A to vertically etch the silicon substrate 11, thereby forming cylindrical elements 14C with respective hourglass heads on the surface of the silicon substrate 11, as shown in FIG. 14(b).
  • a third silicon oxide film 16 having a thickness of about 100 nm is formed by thermal oxidation over the surfaces of the cylindrical elements 14C with respective hourglass heads and of the silicon substrate 11, thereby forming cathodes 17 inside the cylindrical elements 14C with respective hourglass heads.
  • the third silicon oxide film 16 thus formed on the surfaces of the hourglass elements 14C is for sharply tapering the tip portions of the cathodes 17 and enhancing the insulating property of an insulating film underlying the withdrawn electrode, which will be described later.
  • the silicon oxide film formed by thermal oxidation is superior in film quality to a silicon oxide film formed by another method such as vapor deposition, so that it has high insulation resistance. As a result, there can be formed a highly reliable device exhibiting an excellent insulating property during the application of a voltage to the withdrawn electrode, which will be described later.
  • a fourth silicon oxide film 18 used as an insulating film and a conductive film 19 used as the withdrawn electrode are successively deposited by vacuum vapor deposition over the entire surface of the semiconductor substrate 11 including the top surfaces of the silicon oxide masks 12A.
  • ozone gas is introduced so as to form a high-quality silicon oxide film excellent in insulating property.
  • the use of a Nb metal film as the conductive film 19 enables the formation of a uniform withdrawn electrode during a lift-off process, which will be described later.
  • wet etching is performed by using a buffered hydrofluoric acid in an ultrasonic atmosphere to selectively remove the sidewall portions of the cathodes 17 and the silicon oxide masks 12A, thereby lifting off the conductive film 19 deposited on the silicon oxide masks 12A, while exposing the withdrawn electrode 19A having small openings and the cathodes 17.
  • the duration of wet etching is controlled such that the third and fourth silicon oxide films 16 and 18 are overetched. In this manner, the circumferential surfaces of the openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • the surface coating layer 23 composed of the ultra-fine particulate structure is deposited over the entire surface of the silicon substrate 11 including the cathodes 17 by laser ablation, resulting in the field-emission electron source according to the fifth embodiment.
  • the type (undoped-type, p-type, or n-type) and resistivity of the silicon substrate as a target used in laser ablation may be determined in accordance with the preferred properties of the surface coating layer 23.
  • a light source for laser ablation an ArF excimer laser having high energy is preferred.
  • the surface coating layer 23 composed of ultra-fine particles having a desired diameter and deposited in a desired number of layers can be formed over the surface of the cathode 17.
  • an ArF excimer laser beam with a pulse width of 12 nsec and a repetitive frequency of 10 Hz is radiated and focused on a spot of 3 x 1 mm at an energy density of 1 J/cm 2 on a silicon wafer used as the target.
  • the ablation rate for the target composed of the silicon wafer is 0.2 ⁇ m/pulse.
  • the laser ablation is performed under time control, while maintaining a basic degree of vacuum at 0.13 mPa (1 x 10 ⁇ 6 Torr) and introducing He gas at a given flow rate.
  • the surface coating layer 23 composed of the ultra-fine particulate structure constituted by ultra-fine particles having diameters on the order of nanometers can be formed with high reproducibility.
  • the thickness of the surface coating layer 23 composed of the ultra-fine particulate structure formed by laser ablation to be about 10 nm or less, the configuration of the cathode 17 can be reproduced with high fidelity in the surface coating layer 23 so that the tip portion of the surface coating film 23 has a sharply tapered configuration.
  • the method of manufacturing the field-emission electron source according to the fifth embodiment has used laser ablation, there is no possibility that the surface of the cathode 17 suffers damage during the process, while the surface coating layer 23 composed of the uniform ultra-fine particulate structure can be formed without impairing the extremely small configuration of the cathode 17. Furthermore, the manufacturing method enables the formation of an array of extremely small field-emission electron sources at a high density with high precision through the excellently uniform and reproducible process.
  • a low-work-function material other than silicon such as diamond, DLC, or ZrC.
  • the use of the target made of silicon increases the productivity, while the use of the other low-work-function materials lowers the voltage of the operating current.
  • FIG. 6(a) shows a cross-sectional structure taken along the line VI-VI of FIG. 6(b) and FIG. 6(b) shows a plan structure.
  • a withdrawn electrode 19A is formed on a silicon substrate 11 made of a silicon crystal with intervention of an insulating film consisting of upper and lower silicon oxide films 18A and 16A each having circular openings corresponding to regions in which cathodes are to be formed, which have been arranged to form an array.
  • the diameter of the opening of the withdrawn electrode 19A is smaller than the diameters of the respective openings of the upper and lower silicon oxide films 18A and 16A so that the circumferential surfaces of the openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • cathodes 17 each having a cocktail-glass-like configuration composed of a pair of truncated cones with their top surfaces joined to each other and having a side surface including the (331) crystal plane.
  • the upper circumferential edge of the cathode 17 has a sharply sloped cross section with a radius of about 2 nm, which has been formed by crystal anisotropic etching and thermal oxidation process.
  • the portion of the silicon substrate 11 exposed in the respective openings of the upper and lower silicon oxide films 18A and 16A and the cathode 17 have their surfaces coated with a surface coating layer 23 composed of an ultra-fine particulate structure.
  • a material composing the surface coating layer 23 preferably has a low work function so that electrons are emitted positively.
  • silicon particles each having a diameter on the order of nanometers, i.e., a diameter of 10 nm or less are preferred in terms of the efficiency of electron emission.
  • ultra-fine particles composing the surface coating layer 23 are deposited in a single or several layers. In the case where the silicon particle has a diameter of about 10 nm, a single layer of silicon particles is sufficient. In the case where the silicon particle has a diameter of about 5 nm, silicon particles are preferably deposited in two or three layers.
  • the surface coating layer 23 composed of the ultra-fine particulate structure has been formed on the surface of the cathode 17 so that the tip portion of the cathode 17 is prevented from having an obtuse configuration. Accordingly, the radius of curvature of the top portion is not increased nor varied, resulting in easier device design and higher reliability of the device, similarly to the fifth embodiment.
  • the operating current and operating voltage can be reduced, while device characteristics such as operating current and operating voltage do not vary.
  • the ultra-fine particulate structure composing the surface coating layer 23 may be made of a low-work-function material other than silicon, such as diamond, DLC, or ZrC.
  • a first silicon oxide film 12 is formed by thermal oxidation on the (100) crystal plane of the silicon substrate 11 made of a silicon crystal, followed by the deposition of a photoresist film 13 on the first silicon oxide film 12.
  • the photoresist film 13 is subjected to photolithography for forming disk-shaped resist masks 13A each having a diameter of about 0.5 ⁇ m. Subsequently, anisotropic dry etching is performed with respect to the first silicon oxide film 12 by using the resist mask 13A, thereby transferring the pattern of the resist masks 13A to the first silicon oxide film 12 and forming silicon oxide masks 12A therefrom.
  • the removal of the resist masks 13A is followed by anisotropic dry etching performed with respect to the silicon substrate 11 by using the silicon oxide masks 12A, thereby forming cylindrical elements 14A on the surface of the silicon substrate 11.
  • wet etching is performed with respect to the cylindrical elements 14A by using an etching agent having crystal anisotropy, such as an aqueous solution of ethylene diamine and pyrocatechol, thereby forming hourglass elements 14B each having a side surface including the (331) crystal plane and constricted in the middle.
  • an etching agent having crystal anisotropy such as an aqueous solution of ethylene diamine and pyrocatechol
  • second silicon oxide films 15 each having a reduced thickness of about 10 to 20 nm are formed on the sidewalls of the hourglass elements 14B by thermal oxidation.
  • a third silicon oxide film 18 used as an insulating film and a conductive film 19 used as a withdrawn electrode are successively deposited by vacuum vapor deposition over the entire surface of the semiconductor substrate 11 including the top surfaces of the silicon oxide masks 12A.
  • ozone gas is introduced so as to form a high-quality silicon oxide film excellent in insulating property.
  • the use of a Nb metal film as the conductive film 19 enables the formation of a uniform withdrawn electrode during a lift-off process, which will be described later.
  • wet etching is performed by using a buffered hydrofluoric acid in an ultrasonic atmosphere to selectively remove the sidewall portions of the cathodes 17 and the silicon oxide masks 12A, thereby lifting off the conductive film 19 deposited on the silicon oxide masks 12A, while exposing the withdrawn electrode 19A having small openings and the cathodes 17.
  • the duration of wet etching is controlled such that the third and fourth silicon oxide films 16 and 18 are overetched. In this manner, the circumferential surfaces of the openings of the upper and lower silicon oxide films 18A and 16A are recessed relative to the circumferential surface of the opening of the withdrawn electrode 19A.
  • the surface coating layer 23 composed of the ultra-fine particulate structure is deposited over the entire surface of the silicon substrate 11 including the cathodes 17 by laser ablation, resulting in the field-emission electron source according to the sixth embodiment.
  • the surface coating layer 23 composed of the ultra-fine particulate structure constituted by ultra-fine particles each having a desired diameter can be formed on the upper circumferential edge of the top surface of the cathode 17 having a cocktail-glass-like configuration, so that the sharply sloped cross-sectional configuration of the cathode 17 is reflected faithfully in the surface coating layer 23.
  • the surface coating film 23 has a sharply sloped tip portion.
  • the surface coating layer 23 has been formed by laser ablation, there is no possibility that the surface of the cathode 17 suffers damage during the process. Moreover, the excellently uniform and reproducible process enables the formation of an array of extremely small field-emission electron sources at a high density with high precision.
  • the cathode 17 has a tower-like configuration in the fifth embodiment and a cocktail-glass-like configuration in the sixth embodiment, it may have a conical configuration instead.
  • a substrate made of another semiconductor material such as a compound semiconductor of GaAs or the like may be used instead of the silicon substrate 11.

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EP0938122A2 (de) 1999-08-25
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DE69730143D1 (de) 2004-09-09
EP0939418A3 (de) 2000-12-13
EP0802555A3 (de) 1998-05-27
EP0938122A3 (de) 2000-12-13
DE69714123T2 (de) 2002-11-07
EP0802555A2 (de) 1997-10-22
EP0939418B1 (de) 2008-07-02
DE69730143T2 (de) 2004-12-09
EP0938122B1 (de) 2004-08-04
US5925891A (en) 1999-07-20
EP0802555B1 (de) 2002-07-24
US5897790A (en) 1999-04-27
KR980005140A (ko) 1998-03-30
KR100442982B1 (ko) 2004-09-18

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